(a) Field of the Invention
The present invention relates to an objective optical system for endoscopes and, more particularly, to an objective optical system for endoscopes with small diameters such as bronchoscopes.
(b) Description of the Prior Art
Known objectives for bronchoscopes and the like employ a hemispherical lens or a plano-convex lens having a shape approximately hemispherical as shown in FIG. 1. This is because such lenses can be manufactured easily and even the lenses of such degree have sufficient performance for practical use as objectives for endoscopes. However, such objective consisting of such single lens has disadvantages as described below. In case of an objective shown in FIG. 1, the stop S is located between the cover glass L.sub.1 and hemispherical lens L.sub.2, i.e., at a position near the center of curvature of hemispherical lens L.sub.2, in order to prevent astigmatism from occurring. Therefore, the principal rays are scarcely refracted by the spherical surface r.sub.3 of the hemispherical lens L.sub.2 and are refracted only by the plane surface r.sub.1 of the cover glass L.sub.1. Accordingly, as the field angle becomes large, the angle of incidence of ray which enters the image guide G comprising optical fibers becomes large. Optical fibers have such characteristic that their transmission factor becomes small as the angle of incidence of rays which enter them becomes large. This characteristic differs with such factors as the diameter of optical fiber and so forth. Generally, the transmission factor when the angle of incidence is 10.degree. is about 50% of the case that the angle of incidence is 0.degree.. When, therefore, the field angle is made large by making the size of the image surface large, the image in the marginal portion deteriorates. As a method for preventing the above-mentioned disadvantage, the refractive index of the lens may be made large in order to attain large refraction at the first surface of the objective. Another method is to locate the stop at a position as near as possible to the front focal point by tolerating astigmatism to such degree that it is not inconvenient for practical use. However, even when such methods are adopted, the field angle with which an image with favourable quality can be obtained is about 40.degree. to 50.degree. at the maximum.